There's a section in the 1st chapter of the FOTR where Gandalf tells Frodo that "dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself" I take that to mean that while there are still dragons, there aren't any really powerful ones. I don't know if that helps or makes things more confusing.